Transparent conducting oxide for photovoltaic devices

ABSTRACT

One embodiment of the present invention provides a solar cell. The solar cell includes a Si base layer, a passivation layer situated above the Si base layer, a layer of heavily doped amorphous Si (a-Si) situated above the passivation layer, a first transparent-conducting-oxide (TCO) layer situated above the heavily doped a-Si layer, a back-side electrode situated below the Si base layer, and a front-side electrode situated above the first TCO layer. The first TCO layer comprises at least one of: GaInO, GaInSnO, ZnInO, and ZnInSnO.

RELATED APPLICATION

This application is a continuation application of application Ser. No. 13/155,112, entitled “TRANSPARENT CONDUCTING OXIDE FOR PHOTOVOLTAIC DEVICES,” by inventors Jianming Fu, Zheng Xu, Jiunn Benjamin Heng, and Chentao Yu, filed 7 Jun. 2011, which claims the benefit of U.S. Provisional Application No. 61/353,119, entitled “Transparent Conducting Oxide for Photovoltaic Devices,” by inventors Jianming Fu, Zheng Xu, Jiunn Benjamin Heng, and Chentao Yu, filed 9 Jun. 2010.

BACKGROUND

Field

This disclosure is generally related to solar cells. More specifically, this disclosure is related to a solar cell that includes a high work function transparent conducting oxide (TCO) layer.

Related Art

The negative environmental impact caused by the use of fossil fuels and their rising cost have resulted in a dire need for cleaner, cheaper alternative energy sources. Among different forms of alternative energy sources, solar power has been favored for its cleanness and wide availability.

A solar cell converts light into electricity using the photoelectric effect. There are several basic solar cell structures, including a single p-n junction, p-i-n/n-i-p, and multi-junction. A typical single p-n junction structure includes a p-type doped layer and an n-type doped layer. Solar cells with a single p-n junction can be homojunction solar cells or heterojunction solar cells. If both the p-doped and n-doped layers are made of similar materials (materials with equal band gaps), the solar cell is called a homojunction solar cell. In contrast, a heterojunction solar cell includes at least two layers of materials of different bandgaps. A p-i-n/n-i-p structure includes a p-type doped layer, an n-type doped layer, and an intrinsic (undoped) semiconductor layer (the i-layer) sandwiched between the p-layer and the n-layer. A multi-junction structure includes multiple single-junction structures of different bandgaps stacked on top of one another.

In a solar cell, light is absorbed near the p-n junction generating carriers. The carriers diffuse into the p-n junction and are separated by the built-in electric field, thus producing an electrical current across the device and external circuitry. An important metric in determining a solar cell's quality is its energy-conversion efficiency, which is defined as the ratio between power converted (from absorbed light to electrical energy) and power collected when the solar cell is connected to an electrical circuit.

For homojunction solar cells, minority-carrier recombination at the cell surface due to the existence of dangling bonds can significantly reduce the solar cell efficiency; thus, a good surface passivation process is needed. In addition, the relatively thick, heavily doped emitter layer, which is formed by dopant diffusion, can drastically reduce the absorption of short wavelength light. Comparatively, heterojunction solar cells, such as Si heterojunction (SHJ) solar cells, are advantageous. FIG. 1 presents a diagram illustrating an exemplary SHJ solar cell (prior art). SHJ solar cell 100 includes front finger electrode 102, a heavily doped amorphous-silicon (a-Si) emitter layer 104, an intrinsic a-Si layer 106, a crystalline-Si substrate 108, and an Al back-side electrode 110. Arrows in FIG. 1 indicate incident sunlight. Because there is an inherent bandgap offset between a-Si layer 106 and crystalline-Si (c-Si) layer 108, a-Si layer 106 can be used to reduce the surface recombination velocity by creating a barrier for minority carriers. The a-Si layer 106 also passivates the surface of crystalline-Si layer 108 by repairing the existing Si dangling bonds. Moreover, the thickness of heavily doped a-Si emitter layer 104 can be much thinner compared to that of a homojunction solar cell. Thus, SHJ solar cells can provide a higher efficiency with higher open-circuit voltage (V_(oc)) and larger short-circuit current (J_(sc)).

When fabricating solar cells, a layer of transparent conducting oxide (TCO) is often deposited on the a-Si emitter layer to form an ohmic-contact. However, due to the large band gap and high work function of the heavily doped p⁺ amorphous Si emitter layer, it is hard to form low-resistance ohmic contact between a conventional TCO material, such as indium tin oxide (ITO), and the heavily doped a-Si emitter.

SUMMARY

One embodiment of the present invention provides a solar cell. The solar cell includes a Si base layer, a passivation layer situated on a first side of the Si base layer, a layer of heavily doped p-type amorphous semiconductor situated on the passivation layer, a first transparent-conducting-oxide (TCO) layer situated on the heavily doped amorphous semiconductor layer, and a first electrode situated on the first TCO layer. The first TCO layer comprises at least one of: GaInO, GaInSnO, ZnInO, and ZnInSnO.

In a variation on the embodiment, the first side of the Si base layer is facing the incident sunlight.

In a variation on the embodiment, the solar cell includes a second electrode situated on a second side of the Si base layer, and the second side is opposite to the first side.

In a further variation, the second side of the Si base layer is facing the incident sunlight, and the second electrode includes a second TCO layer and a metal grid comprising Cu and/or Ni.

In a variation on the embodiment, the Si base layer includes a crystalline-Si (c-Si) substrate.

In a variation on the embodiment, the Si base layer includes an epitaxially formed crystalline-Si (c-Si) thin film.

In a variation on the embodiment, the passivation layer includes at least one of: undoped a-Si and SiO_(x).

In a variation on the embodiment, the heavily doped p-type amorphous semiconductor layer has a doping concentration between 1×10¹⁷/cm³ and 5×10²⁰/cm³.

In a variation on this embodiment, the first TCO layer has a work function between 4.9 eV and 6.1 eV.

In a variation on the embodiment, the solar cell further comprises a third TCO layer situated on the first TCO layer, and the third TCO layer has a lower resistivity than the first TCO layer.

In a further variation, the third TCO layer includes at least one of: indium tin oxide (ITO), tin-oxide (SnOx), aluminum doped zinc-oxide (ZnO:Al), and Ga doped zinc-oxide (ZnO:Ga).

In a variation on the embodiment, the first electrode comprises at least one of: Ag, Cu, and Ni.

In a variation on the embodiment, the p-type amorphous semiconductor comprises amorphous Si or amorphous Si containing carbon.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 presents a diagram illustrating an exemplary Si heterojunction (SHJ) solar cell (prior art).

FIG. 2 presents a diagram illustrating the band diagrams at the interface between high/medium/low work function TCO material and p-type amorphous Si.

FIG. 3 presents a diagram illustrating the process of fabricating a solar cell in accordance with an embodiment of the present invention.

FIG. 4 presents a diagram illustrating an exemplary solar cell in accordance with an embodiment of the present invention.

FIG. 5 presents a diagram illustrating an exemplary solar cell in accordance with an embodiment of the present invention

In the figures, like reference numerals refer to the same figure elements.

DETAILED DESCRIPTION

The following description is presented to enable any person skilled in the art to make and use the embodiments, and is provided in the context of a particular application and its requirements. Various modifications to the disclosed embodiments will be readily apparent to those skilled in the art, and the general principles defined herein may be applied to other embodiments and applications without departing from the spirit and scope of the present disclosure. Thus, the present invention is not limited to the embodiments shown, but is to be accorded the widest scope consistent with the principles and features disclosed herein.

Overview

Embodiments of the present invention provide an SHJ solar cell that includes a layer of novel TCO material with high work function. The relatively high work function, up to 6.1 eV, of the TCO material ensures lower contact resistance and higher V_(oc).

TCO film has been widely used in solar cells to form ohmic contact with the emitter layer. An SHJ solar cell can be formed by depositing a-Si layers on a c-Si substrate. Note that the a-Si layers include a layer of doped a-Si in order to form a junction with the c-Si substrate or to ensure good electrical contact with a subsequently formed electrode. A TCO layer is often deposited on the doped a-Si layer to form an ohmic contact. However, due to the large band gap and high work function of the p-type doped a-Si layer, it is difficult to find a TCO material with work function that is in alignment with the p-type a-Si in order to minimize the band bending at the TCO and p-type a-Si interface, and to reduce contact resistance and maximize open circuit voltage. For example, the work function of ITO is between 4.5 eV and 4.8 eV. This will cause band bending at TCO and p-type a-Si interface, and make it hard to achieve a low-resistance ohmic contact and high V_(oc). FIG. 2 presents a diagram illustrating the band diagrams at the interface between high/medium/low work function TCO material and p-type amorphous Si. From the band diagram, one can see that, for TCO material with low or medium work function, potential barriers at the interface make it harder for charges (holes) to migrate from the p-type a-Si material to the TCO, thus resulting in higher contact resistance. Hence, it is desirable to use a TCO material that has a relatively high work function.

FIG. 3 presents a diagram illustrating the process of fabricating a solar cell in accordance with an embodiment of the present invention.

In operation 3A, a substrate 300 is prepared. In one embodiment, substrate 300 is a c-Si substrate, which is textured and cleaned. C—Si substrate 300 can be either p-type doped or n-type doped. In one embodiment, c-Si substrate 300 is lightly doped with an n-type dopant, and the doping concentration of c-Si substrate 300 can be between 1×10¹⁶/cm³ and 1×10¹⁷/cm³. Note that other than using c-Si substrate (which is more expensive) as a base layer, it is also possible to deposit a thin c-Si epitaxial film on a relatively cheaper metallurgical-grade Si (MG-Si) substrate to act as a base layer, thus lowering the manufacturing cost. The thickness of the c-Si epitaxial film can be between 5 μm and 100 μm. The surface of c-Si substrate 300 can be textured to maximize light absorption inside the solar cell, thus further enhancing efficiency. The surface texturing can be performed using various etching techniques including dry plasma etching and wet etching. The etchants used in the dry plasma etching include, but are not limited to: SF₆, F₂, and NF₃. The wet etching etchant can be an alkaline solution. The shapes of the surface texture can be pyramids or inverted pyramids, which are randomly or regularly distributed on the surface of c-Si substrate 300.

In operation 3B, a passivation layer 304 is deposited on top of c-Si substrate 300. Passivation layer 304 can significantly reduce the density of surface carrier recombination, thus increasing the solar cell efficiency. Passivation layer 304 can be formed using different materials such as intrinsic a-Si or silicon-oxide (SiO_(x)). In one embodiment, a layer of intrinsic a-Si is deposited on c-Si substrate 300 to form passivation layer 304. Techniques used for forming passivation layer 304 include, but are not limited to: PECVD, sputtering, and electron beam (e-beam) evaporation. The thickness of passivation layer 304 can be between 3 nm and 10 nm.

In operation 3C, a heavily doped p-type doped amorphous semiconductor layer is deposited on passivation layer 304 to form an emitter layer 306. The p-type amorphous semiconductor can be a-Si or amorphous SiC (a-SiC). In one embodiment, emitter layer 306 includes a-Si. The doping concentration of emitter layer 306 can be between 1×10¹⁷/cm³ and 5×10²⁰/cm³. The thickness of emitter layer 306 can be between 3 nm and 10 nm. Techniques used for depositing emitter layer 306 include PECVD. Because the thickness of emitter layer 306 can be much smaller compared with that of the emitter layer in a homojunction solar cell, the absorption of short wavelength light is significantly reduced, thus leading to higher solar cell efficiency.

In operation 3D, a layer of high work function TCO material is deposited on top of emitter layer 306 to form TCO layer 308. Compared with conventional TCO material, such as ITO, used in solar cells, TCO layer 308 includes TCO material with a relatively higher work function. In one embodiment, the work function of TCO layer 308 is between 4.9 eV and 6.1 eV. Examples of high work function TCO include, but are not limited to: GaInO (GIO), GaInSnO (GITO), ZnInO (ZIO), ZnInSnO (ZITO), their combinations, as well as their combination with ITO. Techniques used for forming TCO layer 308 include, but are not limited to: PECVD, sputtering, and e-beam evaporation. Note that in addition to providing low-resistance ohmic contact, the higher work function of TCO layer 308 can also result in a higher V_(oc).

In operation 3E, metal front electrodes 310 are formed on top of TCO layer 308. Front metal electrodes 310 can be formed using various metal deposition techniques at a low temperature of less than 300° C. In one embodiment, front electrodes 310 are formed by screen-printing Ag paste. In another embodiment, front electrodes 310 are formed by electroplating Cu and/or Ni.

In operation 3F, a back electrode 302 is formed on the opposite side to the front side. In one embodiment, the back electrode stack can include a passivation layer, an n-typed heavily doped semiconductor layer, a TCO or a metal layer with relatively low work function (such as between 4.0 eV and 5.0 eV), and a metal grid.

After the formation of front electrodes 310 and back electrode 302, various techniques such as laser scribing can be used for cell isolation to enable series interconnection of solar cells.

Although adopting high work function TCO material can result in lower contact resistance between TCO layer 308 and emitter layer 306, high work function TCO material tends to have a larger resistivity than that of the ITO. For example, an ITO material that has 5% tin oxide has a low resistivity of 200 μΩ·cm, which is much smaller than that of the high work function TCO materials. Hence, to reduce the overall resistance, TCO layer 308 may be a bi-layer structure that includes a high work function TCO sub-layer and an ITO sub-layer.

FIG. 4 presents a diagram illustrating an exemplary solar cell in accordance with an embodiment of the present invention. Solar cell 400 includes a base layer 402, a passivation layer 404, an emitter layer 406, a TCO layer 408, a back-side electrode 410, and a front-side metal grid 412.

Base layer 402 can be a c-Si substrate or an epitaxially formed c-Si thin film. Passivation layer 404 can be an oxide layer or a layer of intrinsic a-Si. Emitter layer 406 can be either p-type doped or n-type doped. In one embodiment, emitter layer 406 is p-type doped a-Si. TCO layer 408 includes two sub-layers 408-1 and 408-2. Sub-layer 408-1 is on top of emitter layer 406. To ensure a good ohmic contact with a low contact resistance, in one embodiment, sub-layer 408-1 is formed using high work function TCO material, including, but not limited to: GaInO (GIO), GaInSnO (GITO), ZnInO (ZIO), ZnInSnO (ZITO), and their combinations. Sub-layer 408-2 includes TCO materials having low resistivity, such as ITO, tin-oxide (SnO_(x)), aluminum doped zinc-oxide (ZnO:Al), or Ga doped zinc-oxide (ZnO:Ga). Back-side electrode can include a passivation layer, an n-typed heavily doped semiconductor layer, a TCO or a metal layer with relatively low work function (such as that between 4.0 eV and 5.0 eV), and a metal grid. Front-side metal grid 412 can include screen-printed Ag grid or electroplated Cu and/or Ni grid.

In addition to be deposited on the front side (the side facing the sun) of the solar cell, the high work function TCO layer can also be used on the side opposite to the incidence of sunlight. In one embodiment, the passivation layer and the heavily doped p-type semiconductor layer are deposited on the back side of the c-Si base layer, facing away from incident light. The high work function TCO layer is then deposited on the back side as well. The electrode on the front side of the solar cell includes a TCO layer with lower work function, such as ITO. The solar cell performance can still benefit from the low ohmic contact resistance between the high-work function TCO and the heavily doped p-type semiconductor layer.

FIG. 5 presents a diagram illustrating an exemplary solar cell in accordance with an embodiment of the present invention. Solar cell 500 includes a base layer 502, passivation layers 504 and 506, an emitter layer 508, a BSF layer 510, TCO layers 512 and 514, a back-side electrode 516, and a front-side electrode 518.

Base layer 502 can be lightly doped c-Si. In one embodiment, base layer 502 is p-type doped. Passivation layers 504 and 506 can include an intrinsic a-Si or oxide layer or a combination thereof. Emitter layer 508 can be heavily doped n-type amorphous semiconductor, and BSF layer 510 can be heavily doped p-type amorphous semiconductor, such as a-Si or a-SiC. Front-side TCO layer 512 interfaces with n-type doped emitter layer 508, and includes low work function TCO material, such as ITO. Back-side TCO layer 514 interfaces with p-type doped BSF layer 510, and includes high work function TCO material, such as GIO, GITO, ZIO, ZITO, and their combinations. Back-side electrode 516 and front-side electrode 518 are similar to the ones shown in FIG. 4.

Note that it is also possible to place the heavily doped p-type emitter on the back side of the solar cell with a lightly doped n-type base layer, and to include a front surface field (FSF) layer. As long as the TCO material interfacing with heavily doped p-type material has a relatively high work function, the overall performance of the solar cell can benefit from the reduced ohmic contact resistance between the TCO and the heavily doped p-type material.

The foregoing descriptions of various embodiments have been presented only for purposes of illustration and description. They are not intended to be exhaustive or to limit the present invention to the forms disclosed. Accordingly, many modifications and variations will be apparent to practitioners skilled in the art. Additionally, the above disclosure is not intended to limit the present invention. 

What is claimed is:
 1. A photovoltaic structure, comprising: a Si base layer; a p-type doped amorphous Si layer positioned on a first side of the Si base layer; an n-type doped semiconductor layer positioned on a second side of the Si base layer; a first transparent-conductive-oxide layer positioned on and in direct contact with the p-type doped amorphous Si layer, wherein the first transparent-conductive-oxide layer has a work function that is greater than 5.7 eV and less than to 6.1 eV, thereby reducing potential barriers for carriers at an interface between the p-type doped amorphous Si layer and the first transparent-conductive-oxide layer; and a second transparent-conductive-oxide layer positioned on and in direct contact with the n-type doped semiconductor layer, wherein the second transparent-conductive-oxide layer has a work function that is lower than the work function of the first transparent-conductive-oxide layer.
 2. The photovoltaic structure of claim 1, further comprising a first metallic grid positioned on the first transparent-conductive-oxide layer, wherein the first metallic grid comprises an electroplated layer that includes Cu.
 3. The photovoltaic structure of claim 1, further comprising a metallic grid positioned on the second transparent-conductive-oxide layer, wherein the metallic grid comprises an electroplated layer that includes Cu.
 4. The photovoltaic structure of claim 1, wherein the Si base layer includes a crystalline-Si substrate or an epitaxially formed crystalline-Si thin film.
 5. The photovoltaic structure of claim 1, further comprising at least one of: a first passivation layer positioned between the Si base layer and the p-type doped amorphous Si layer; and a second passivation layer positioned between the Si base layer and the p-type doped amorphous Si layer.
 6. The photovoltaic structure of claim 5, wherein the first passivation layer and the second passivation layer include at least one of: undoped amorphous Si and SiO_(x).
 7. The photovoltaic structure of claim 1, wherein the p-type doped amorphous Si layer comprises amorphous Si or amorphous Si containing carbon, and wherein the p-type doped amorphous Si layer has a doping concentration between 1×10¹⁷/cm³ and 5×10²⁰/cm³.
 8. The photovoltaic structure of claim 1, further comprising a third transparent-conductive-oxide layer positioned on the first transparent-conductive-oxide layer, wherein the third transparent-conductive-oxide is selected from a group consisting of: indium tin oxide (ITO), tin-oxide (SnO_(x)), aluminum doped zinc-oxide (ZnO:Al), Ga doped zinc-oxide (ZnO:Ga), and any combination thereof.
 9. The photovoltaic structure of claim 1, wherein the second transparent-conductive-oxide layer comprises indium tin oxide (ITO). 